1. Field of the Invention
The present invention relates generally to integrated circuits having IGFET and bipolar devices and more specifically to an IC having junctional isolated bipolar devices in an unisolated IGFET IC.
2. DESCRIPTION OF THE PRIOR ART
In an effort to increase speed and current capacity, while reducing power dissipation, integrated circuits are being designed incorporating insulated gate field effect transistors (IGFET) and bipolar devices. Early processes generally formed junction field effect transistors and bipolar transistors in junction isolated regions as described in U.S. Pat. Nos. 3,576,475 and 3,595,715. Because the circuit used junction field effect transistors, the field effect transistors and bipolar transistors had to be formed in junction isolated islands. This increased the space required to form the circuits. In an effort to reduce the space required, as well as to provide a process for compatibly forming IGFETs and bipolar transistors, U.S. Pat. No. 3,865,649 formed the devices in dielectrically isolated regions. This patent requires a specific sequence of preparing the material in the dielectrically isolated regions to provide device characteristics required. This processing is considered complicated and expensive and does not take advantage of the self-isolating characteristic of IGFET devices. The use of the self-isolating characteristic of IGFETs in combination with the self-isolating characteristic of emitter followers are illustrated in U.S. Pat. Nos. 3,609,479 and 3,636,372. The circuits of these two patents are limited to specific environments and configurations since no isolated regions are provided for other devices. Thus, although taking advantage of the self-isolating characteristic of IGFETs to minimize space, it does not also provide in the structure junction isolation for a single bipolar device.
Thus, there exists a need for a process of fabricating an integrated circuit which bridges the gap between the totally isolated and the totally unisolated integrated circuits having IGFET and bipolar transistors.